Field of the Invention
The invention relates to an electro-optic modulator and to an electro-optic distance measuring.
Description of Related Art
U.S. Pat. Nos. 5,129,017, 5,050,948, 5,138,480, 5,278,924, WO02097526 and JP 9236783 A disclose integrated (coplanar waveguide or CPW) optics modulators with travelling wave electrodes. The modulators are structured as Mach-Zehnder-Modulators, that is, a beam of light is split up into two parts, one part of the light is phase modulated, the two parts are rejoined, and as a result the amplitude of the rejoined parts is modulated in accordance with the phase modulation. The light takes a single pass through the modulator, that is, in one direction only.
Investigations on short path length high speed optical modulators in LiNbO3 with resonant type electrodes, R. Krahenbiihl, M. M. Howerton, Journal of Lightwave Technology, Vol. 19, No. 9 pp. 1287-1297 (2001) describes different types of resonant electrode structures with the goal of reducing the active electrode length and enhance modulation efficiencies. Electrode topologies shown are limited to Mach-Zehnder-Modulators.
High-sensitivity lumped-element bandpass modulators in LiNbO3, G. E. Betts, L. M. Johnson, C. H. Cox, IEEE/OSA Journal of Lightwave Technology, Vol. 7, no. 12, pp. 2078-2083, December 1989, describes bandpass modulators based on an integrated Mach-Zehnder-Modulator.
Design and performance of the bidirectional optical single-sideband modulator, A. Loayssa et. al., Journal of Lightwave Technology, Vol 21, No. 4, pp. 1071-1082, April 2003, present a study of a bidirectional optical single-sideband modulator used to achieve optical single-sideband modulation that uses a standard single-electrode Mach-Zehnder modulator and passive fiber-optic components. The electrooptic modulator is operated by driving the radio frequency electrode bidirectionally, that is, with one signal each fed into opposing ends of the same electrode.
FIG. 1 schematically shows a distance measuring device for measuring absolute distance according to the prior art, wherein a light source 101 emits light, typically in the visible or infrared range, with center wavelength λ, the spectral width Δλ of the source being broad enough in order to ensure a low coherence light emission. The parallel light beam emitted by the broadband source 101 illuminates a polarising beam splitter 102, which ensures a linear polarization state for one of the transmitted beams. The polarized beam passes through an electro-optic crystal 103 having electrodes 104 on opposite sides. The electrodes 104 allow application of an electric field parallel to one of the main crystallographic axis of the electro-optic crystal 103. A sinusoidal electric signal with a frequency f is generated by a signal source 108 and applied to the electrodes 104. This electric field generates a modification of the refractive index difference between the slow and the fast optical axes of the crystal. A phase modulation is thus introduced between the two orthogonal waves.
At the output of the electro-optic crystal 103, a quarter wave plate 105 is placed with its axes oriented at 45° with respect to the main axes of the electro-optic crystal 103. The light beam after passing through the quarter wave plate 105 passes on, along the distance to be measured, to reach a target. A corner mirror 106 or other reflecting element is fixed to the target, reflecting the light back to the optical source. After passing a second time through the quarter wave 105 plate, the two orthogonal waves of the returning light are rotated by 90° and cross the electro-optic crystal 103 a second time, now in the opposite direction. A resulting beam, modulated in amplitude according to the interference of outgoing and returning light, is captured by a photoreceiver 107.
Relevant distance-measuring devices according to this principle are known from EP 0 205 406, EP 0 313 518, EP-A-1 647 838, WO 97/18486 and EP patent application number 10 405 078, for example. The content of these applications is incorporated in its entirety by reference for elucidating the functioning of the Fizeau method for absolute distance measurement.
Basically, a light beam, from a laser or from a broadband light source, is generated, and guided by a focusing optical unit onto a polarizing beam splitter for linearly polarizing the light, and is subsequently guided onto a measurement path by an electro-optical modulator, a lambda/4 retarder and an exit optical unit. Light returning along the measurement path passes through the elements mentioned as far as the polarizing beam splitter and is guided onto a detector by the latter. An evaluation unit serves for determining the length of the measurement path on the basis of the detector signal.
What is of importance in the present context is that, in this method, outgoing and returning measurement light is modulated in a modulator. By variation of the frequency of the modulation, a minimum of the intensity of a detected measurement light beam is determined (or substantially synonymously, a zero-crossing of the derivative of the intensity). The length of the measurement path between the measurement device and a retroreflector or a semi-cooperative target is determined from the minimum frequency. A semi-cooperative target returns at least part of incident light along the direction of the incident light, e.g. by diffuse reflection.
Current implementations of Fizeau-principle based distance-measuring devices use electro-optic modulators with bulk crystals exhibiting the Pockels-effect. In order to reach the voltages of several 100 V (over a crystal width of ˜1 mm) required for full modulation, the modulator needs an electrical drive-power of ˜1 W, and the crystal is placed in a electrical resonator. Setting a particular modulation frequency requires mechanical tuning of the resonator, thus limiting the measurement rate (to e.g. 20 Hz).
It is desirable to speed up the measurement by using an integrated optics modulator in a distance measurement device. However, since the measurement principle of the distance measurement device requires the light to pass the modulator twice, in opposing directions, known single pass modulators are not suitable.